Study On Nonlinear Shear Modulus Of Arterial Vessels

Cardiovascular diseases are the leading cause of death in China,and the number of cardiovascular diseases and deaths is still on the rise.This leads to an increasing burden of cardiovascular disease in our country.It has become a major public health problem in our country.Atherosclerosis is a major cause of many cardiovascular diseases.Atherosclerosis has been shown to alter the mechanical parameters of vascular tissue.Therefore,non-invasive methods to characterize the mechanical properties of blood vessels are very important for the diagnosis and treatment of cardiovascular diseases.Ultrasonic shear wave elastography is a non-invasive imaging technique that induces and detects shear wave propagation in tissues by using acoustic radiation force.However,up to now,the advanced techniques in this field mainly focus on the estimation of the second order elastic modulus.In order to better understand and diagnose the pathological changes of vascular tissue,multi-parameter diagnosis is desirable,which also requires the measurement of additional biological tissue parameters.The nonlinear shear modulus can supplement the measurement of shear modulus and be helpful for early diagnosis.The measurement of nonlinear shear modulus also reduces the confounding effect of ultrasonic shear wave elastography.This project mainly studies the use of ultrasonic shear wave elastography technology to estimate the initial shear modulus μ₀ and nonlinear shear modulus of blood vessels A,and verifies the feasibility of nonlinear shear modulus to evaluate the vascular mechanical properties through A multi-angle comparative study.In this project,the initial shear modulus μ₀ and nonlinear shear modulus A are estimated,respectively,by the relationship between stress and shear modulus and the relationship between strain and shear modulus.In this study,not only the transmural pressure of blood vessels,but also the circumferential stress in the transverse direction of blood vessels were considered.This study was based on Verasonics^TM Vantage ^TM256 ultrasonic system to build an experimental platform and conduct data acquisition and processing according to a certain time sequence.The focusing force of the ultrasonic probe is controlled to generate acoustic radiation and act on the vessel wall to generate and propagate shear wave.The plane wave with high frame rate is transmitted to obtain the shear wave propagation data.The propagation data of shear waves were obtained in the longitudinal and transverse directions of the vessels,and then the group velocity of the shear waves and the thickness radius of the vessels were calculated.The group velocity is used to estimate the second order shear modulus and relationship between stress and shear modulus is used to estimate the initial shear modulus and the nonlinear shear modulus A.On the other hand,strain is calculated based on the variation of vascular thickness,and the relationship between strain and shear modulus is used to estimate the initial shear modulus and the nonlinear shear modulus A.In this study,the stiffness of vascular phantom was controlled by the number of freezing-thawing cycles（F/T）of PVA material,which were divided into three groups of 3F/T,4F/T and 5F/T accordingly.Water column pressure was used to simulate the environment of human blood pressure,and the variation range was divided into60～90mmHg and 90～120mmHg.Based on the stress and strain estimation of the initial shear modulus and the nonlinear shear modulus A,the results showed that there were significant differences between the initial shear modulus and the nonlinear shear modulus A.The results show that the measurement of the nonlinear shear modulus can distinguish the vascular tissues with different stiffness.The results are compared under three conditions:radial direction and intracavity pressure,transverse direction and intracavity pressure,radial direction and circumferential stress.The results showed that there were significant differences among the three conditions.The measurement of nonlinear shear modulus also reflects the anisotropy of vascular tissue to some extent.Finally,by comparing the results of stress and strain angles,it is found that the results of stress and strain angles are significantly different,and the reasons are discussed.The accurate measurement of thickness radius is very important and is also a research direction in the next stage.The results of this study show that the use of nonlinear shear modulus to evaluate the mechanical properties of blood vessels can eliminate the interference of blood pressure to the measurement of the mechanical properties of blood vessels,and can effectively reflect the differences in the mechanical properties of blood vessels.The measurement of nonlinear shear modulus can also reflect the anisotropy of blood vessels in a certain distance.Nonlinear shear modulus can complement the measurement of shear modulus and is expected to be a diagnostic index for evaluating vascular biomechanical properties in the future.